Recent studies showed that intestinal microbiota consist more than thousand bacteria, and these microorganisms play the role as balance and continuity of the anatomical and functional integrity gastrointestinal tract. Chronic diseases, nutritional factors, lifestyle factors and medications including antibiotics changes the host microbiota in short or long-term period. Antibiotics are the most commonly used drugs in pediatrics routine practice. Although frequency and severity of side effects alter the choice of drug, mild complications may be taken into consideration for treatment by a physician in the profit and loss balance. Our study group (PROBAGE Working Group) have been recently showed that Lactobacillus reuteri DSM 17938 reduced the duration of diarrhea in children with acute infectious diarrhea, and are safe and well-tolerated. In this study the investigators aim to evaluate the potential effect of Lactobacillus reuteri DSM 17938 on the prevention of antibiotic associated diarrhea in children.